For the plan to be the most successful, SW free foods are a mixture of low fat and high filling foods combined with a 1/3 superfree.
However using free foods in a cake as a replacement to flour or cooking something to a hard type snack (potato crisps and lasagne crisps) would most likely change their satiety making them easy to over consume and less filling. Think of the cooked, pureed, blended fruit must be synned rule. This is because it becomes less satisfying in that form and you need more of it.
It is when free foods are used in this way that slimming world state we should syn these foods. This is to protect our losses.
Although slimming world is not a calorie controlled diet, at the end of the day it does come down to the fact that if we eat over the calories our body physically needs in a day we won't lose weight and may even gain.
Slimming world cleverly designed their plan so that we dont have to weigh foods or count calories. By giving us a list of low fat but filling free foods and advising a 1/3 superfree.
However if you find you are still losing weight by eating those kind of snacks, like cous cous cake, chick pea cake, SW crisps (made from lasagna sheets/potatoes) etc then carry on. But you know those are the areas to stop if your weight loss stops.
It is worth bearing in mind for those that can eat those things and still lose weight that it is probably because you are still under your daily recommended calorie intake. However if you are having huge portions at meals and then snacking excessively on these kinds of snacks as well with no restraint then the likelihood is you will be way over the calories you burn, so it should come as no surprise to find weight not coming off or seeing a small gain on those scales. Also dont forget as your weight comes off, you require less daily calories, so while it might work for you now to eat those kinds of things and still lose weight, it is very likely it will come to a point that eating these unfilling snacks will eventually affect your weight loss.
I hope that explains it all for you